Filter devices of this kind having a variety of different designs and with exchangeable filter elements that conform to various specifications are prior art. The filter elements are used in such filter devices for maintenance of fluids, in particular in oil-hydraulic systems. In order to guarantee the operational reliability of such units, which is largely dependent on the faultless performance of the filter devices, such filter elements are commonly monitored with respect to their dirt retention capability by differential pressure measurement (DE 10 2005 043 752 B3) so as to avoid the risk of breakdowns or even damage occurring due to overloading, which can result in significant economic losses in the case of expensive systems.